The present invention relates to pneumatic braking systems for tractor-trailer combinations and more particularly to a system including improved control pressure apparatus for providing braking of the trailer to be more nearly in coincidence with that of the tractor and in a preferred form to be slightly in advance of tractor braking.
In conventional tractor-trailer brake systems it is known to control the operation of the trailer brake actuators in response to the magnitude of pressure in a service line at the tractor; the service line transmits pressure to the tractor brake actuators with the magnitude being varied by a manually operated foot or treadle valve located on the tractor. The pressure in the service line is used as a pressure signal for actuation of an emergency relay valve; in some applications the emergency function from the valve is eliminated and the valve performs only a relay function. The latter valve operates in conjunction with an air supply on the trailer to provide pressure from the trailer air supply to the trailer brake actuators in response to the pressure signal and having a magnitude varying in accordance with the magnitude of the signal. However, there is a significant time lag from the time at which the pressure in the service line reaches and actuates the tractor brake actuators and the time at which the pressure signal reaches and actuates the emergency relay valve to actuate the trailer brake actuators. In addition to the time delay there normally is a significant reduction in pressure between the tractor and the emergency relay valve at the trailer. Thus the tractor brakes often engage substantially ahead of the trailer brakes and may result in trailer overrun or "jackknifing."
It is a practice to provide the tractor with a hand operated valve for controlling the actuation of the trailer brakes so that the operator can apply the trailer brakes independently of the tractor brake; this structure, however really does not solve the problem since it would require extreme dexterity of the operator to actuate both the hand valve and foot valve in some reasonable time sequence in an emergency condition in order to assure trailer braking with or before tractor braking.
In the present invention the trailer pressure control apparatus is mounted to the tractor proximate the rear thereof. The control apparatus includes a fast-acting auxiliary amplifying relay valve having an auxiliary fluid pressure reservoir directly connected to and mounted proximate thereto. As will be seen in one form of the invention the pressure control apparatus takes the place of and hence eliminates the need for the conventional tractor protection valve (the tractor protection valve is operable in response to a substantial drop in air pressure to dump the air in the line to the pressure supply in the trailer; at the same time the emergency relay valve blocks the trailer supply from the line and applies pressure to the trailer brakes).
The fast acting amplifying valve reduces the time for a pressure signal of sufficient magnitude to reach the emergency relay valve at the trailer to actuate the trailer brakes and at the same time the amplifying valve will amplify the signal to more than compensate for the inherent pressure drops in the system.
The result is that the trailer brakes will be actuated faster than with conventional systems and at the same time can be actuated to lead the tractor brakes. The system can be used with multi-trailer applications and in these applications will provide for sequential actuation of the brakes from the rearmost trailer brakes up to the tractor.
Therefore it is an object of the present invention to provide a tractor-trailer braking system including pressure control apparatus for the trailer whereby the trailer brakes will be actuated co-incidentally with or before the tractor brakes.
It is another object of the present invention to provide such a system for use with a series of trailers and in which the brakes of tractor and trailers will be actuated sequentially from the rearmost trailer brakes up to the tractor.